This invention relates generally to floating fasteners in assembled parts and, more specifically, to tolerancing of such floating fasteners.
Two or more parts may be fastened together using a fastener extending through mating apertures in the parts. Since the fastener is typically smaller in cross-section than the apertures, a clearance exists between the fastener and aperture which allows the fastener to float radially until it is secured in position.
A typical aperture is circular and is usually formed by being drilled through the mating parts. The typical fastener is in the form of a cylindrical bolt threaded on at least one end for being tightened in tension across the parts using a cooperating nut. The fastener and apertures may have other cross-sectional configurations as desired.
Maximum joint strength may be obtained by maximizing the diameter of the fastener and minimizing the radial clearance in the mating holes. However, this becomes more difficult as a series of mating apertures and corresponding fasteners are required. For example, two parts may be joined together using an annular array of fasteners extending through a corresponding annular array of mounting holes arranged in mating pairs in two or more parts. The bolt holes are therefore arranged annularly.
The bolt-hole circle joining arrangement is commonly found in both stationary and rotating parts. A typical stationary part example is the joining of two sections of pipe at corresponding radial end flanges through which an array of mounting bolts are tightened to form a compression joint.
Exemplary rotating parts may be found in rotor components of a gas turbine engine such as rotor disks and cooperating driveshafts therein which require accurate or precise symmetry for maximizing initial balance for operating at relatively high rotational speeds.
The bolt-hole circle on each part of the assembly is typically formed with mounting holes of uniform or equal nominal diameter. The holes are typically positioned at a common nominal radius from the axial centerline axis of the parts and spaced apart on centers at a uniform nominal circumferential spacing.
During assembly, the bolt-hole circles of the adjoining parts are suitably aligned with each other so that the individual fasteners may be inserted through corresponding pairs of the mating holes. The fasteners usually have a common nominal outer diameter which is maximized so as to completely fill each of the mating holes with minimal radial clearance. The maximum fastener size and minimum radial clearance improve the strength of the assembled joint and reduce the inherent unbalance of the joint.
Because the position and size of the mounting holes, and the size of the fasteners, necessarily vary statistically during manufacture, they are specified with a desired nominal value and with a specific variation or tolerance typically expressed in positive and negative mils which determine the precision requirements of the manufacturing equipment. Small tolerances require correspondingly higher precision in manufacturing, typically effected at greater cost and manufacturing effort. Large tolerances allow use of less precise manufacturing equipment with a corresponding reduction in manufacturing cost and effort.
Since gas turbine engine components require precision manufacture, precision manufacturing equipment is normally required for accurately forming both the fasteners and their mounting holes in the adjoining parts. Nevertheless, each of the many fasteners, and each of the many mounting holes in the adjoining parts, is individually subject to statistical variation in size and position. An exemplary statistical variation is the common bell or gaussian curve, although others are also known. Variation in position and size may be greater than or less than the nominal or optimum values.
Floating fastener joint design becomes even more complex as the number of fasteners and corresponding mating holes increases. In a typical gas turbine engine, tens of fasteners are used in a typical floating fastener assembly of two or more adjoining annular parts. The high number of mating holes substantially increases the likelihood of misalignment between one or more of the mating hole locations. Misalignment at any one location causes a radial overlap between mating holes which effectively decreases the available diameter through which the fastener may be inserted. Inability to insert even one of the many fasteners through its respective mating hole, due to interference with the hole, is unacceptable.
It is common, however, to cool an interfering fastener, with dry ice for example, to temporarily reduce its diameter so as to minimize or reduce the interference with the misaligned mating hole and complete the assembly. If this is unsuccessful, the joint must be disassembled and the parts re-indexed in an attempt to obtain complete assembly of all of the fasteners through respective mating holes. Alternatively, replacement parts or fasteners must be used.
In order to reduce the likelihood of joint misalignment due to one or more floating fasteners in a bolt-hole circle, the differences in diameter between fasteners and their mating holes may be suitably increased, but at the expense of an undesirable increase in clearance therebetween. Alternatively, different ranges of variation in position and size may be specified in an attempt to reduce the likelihood of misalignment. However, these solutions are based primarily on previous manufacturing experience which is typically of limited extent and thus ineffective for optimizing the floating fastener joints.
Accordingly, it is desired to provide a method of tolerancing floating fasteners and mating apertures which improves assembly of a plurality of parts.